Formation process for lithium-ion batteries

ABSTRACT

A method for producing a battery includes providing a battery having a positive electrode, a negative electrode, and an electrolyte that includes a solvent and a salt. The capacity of the negative electrode is less than that of the positive electrode and the negative electrode includes an active material having an average potential versus a lithium reference electrode of greater than approximately 0.2 volts. The method also includes applying an initial charge to the battery at a voltage that is greater than a fully charged voltage of the battery for a sufficient amount of time to cause at least a portion of the solvent to undergo a reduction reaction. The step of applying an initial charge to the battery acts to increase the irreversible capacity loss of the battery during the initial charge and provides the battery with enhanced tolerance to deep discharge conditions.

BACKGROUND

The present application relates generally to the field of lithium-ionbatteries or cells. More particularly, the present application relatesto improved methods for initially charging (i.e., forming) suchbatteries.

Lithium-ion batteries or cells include one or more positive electrodes,one or more negative electrodes, and an electrolyte provided within acase or housing. Separators made from a porous polymer or other suitablematerial may also be provided intermediate or between the positive andnegative electrodes to prevent direct contact between adjacentelectrodes. The positive electrode includes a current collector havingan active material provided thereon, and the negative electrode includesa current collector having an active material provided thereon. Theactive materials for the positive and negative electrodes may beprovided on one or both sides of the current collectors.

FIG. 1 shows a schematic representation of a portion of a lithium-ionbattery 10 such as that described above. The battery 10 includes apositive electrode 20 that includes a positive current collector 22 anda positive active material 24, a negative electrode 30 that includes anegative current collector 32 and a negative active material 34, anelectrolyte material 40, and a separator (e.g., a polymeric microporousseparator, not shown) provided intermediate or between the positiveelectrode 20 and the negative electrode 30. The electrodes 20, 30 may beprovided as relatively flat or planar plates or may be wrapped or woundin a spiral or other configuration (e.g., an oval configuration). Theelectrode may also be provided in a folded configuration.

During charging and discharging of the battery 10, lithium ions movebetween the positive electrode 20 and the negative electrode 30. Forexample, when the battery 10 is discharged, lithium ions flow from thenegative electrode 30 to the positive electrode 20. In contrast, whenthe battery 10 is charged, lithium ions flow from the positive electrode20 to the negative electrode 30.

Once assembly of the battery is complete, an initial charging operation(referred to as a “formation process”) may be performed. During thisprocess, a stable solid-electrolyte-inter-phase (SEI) layer is formed atthe negative electrode and also possibly at the positive electrode.These SEI layers act to passivate the electrode-electrolyte interfacesas well as to prevent side-reactions thereafter.

One issue associated with conventional lithium-ion batteries relates tothe ability of the batteries to withstand repeated charge cycling thatinvolves discharges to near-zero-volt conditions (so-called “deepdischarge” conditions). This deep discharge cycling may decrease theattainable full charge capacity of the batteries, which is known in theart as capacity fade. For example, a battery that initially is chargedto 2.8 volts (V) may experience capacity fade with repeated deepdischarge cycling such that after 150 cycles the full charge capacity ofthe battery is much less than the initial capacity.

One consequence of capacity fade in rechargeable batteries is that thebatteries will require increasingly frequent charging as the capacityfade progresses. In certain circumstances, this may be relativelyinconvenient for the user of the batteries. For example, certainimplantable medical devices may utilize rechargeable batteries as apower source. Increasing capacity fade will require the patient to morefrequently charge the rechargeable batteries.

Accordingly, it would be advantageous to provide a rechargeable battery(e.g., a lithium-ion battery) with increased resistance to capacity fadefor batteries that experience repeated deep discharge cycling.

SUMMARY

An exemplary embodiment relates to a method for producing a batteryincludes providing a battery having a positive electrode, a negativeelectrode, and an electrolyte that includes a solvent and a salt. Thecapacity of the negative electrode is less than that of the positiveelectrode and the negative electrode includes an active material havingan average potential versus a lithium reference electrode of greaterthan approximately 0.2 volts. The method also includes applying aninitial charge to the battery at a voltage that is greater than a fullycharged voltage of the battery for a sufficient amount of time to causeat least a portion of the solvent to undergo a reduction reaction. Thestep of applying an initial charge to the battery acts to increase theirreversible capacity loss of the battery during the initial charge andprovides the battery with enhanced tolerance to deep dischargeconditions.

Another exemplary embodiment relates to a method for manufacturing alithium-ion battery that includes providing a battery having a positiveelectrode, a negative electrode, and an electrolyte. The negativeelectrode is configured such that the battery is limited by the capacityof the negative electrode and includes an active material having anaverage potential versus a lithium reference electrode of greater thanapproximately 0.2 volts. The method also includes charging the batteryduring a formation process with a voltage that is greater than a fullycharged voltage of the battery. During the formation process, thevoltage of the negative electrode versus a lithium reference electrodedrops to a level where a reduction reaction occurs for at least aportion of the electrolyte. The formation process provides the batterywith improved tolerance to overdischarge conditions such that theoccurrence of capacity fade with repeated overdischarge cycles isreduced.

Another exemplary embodiment relates to a method for producing a batterythat includes applying an initial charge to a lithium-ion battery thatcomprises an electrolyte, a positive electrode, and a negativeelectrode. The negative electrode has a lower capacity than the positiveelectrode and includes a negative active material that has an averagepotential of greater than approximately 0.2 volts versus a lithiumreference electrode. The step of applying an initial charge utilizes avoltage that is greater than a fully charged voltage of the battery andis performed for a sufficient amount of time to form a film at thenegative electrode that results from the reduction of the electrolyte.The zero volt crossing potential for the battery is increased to a levelgreater than the decomposition potential of an active material providedon the positive electrode to provide enhanced tolerance to repeatedoverdischarge conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a portion of a lithium-ion batteryaccording to an exemplary embodiment.

FIG. 2 is a schematic cross-sectional view of a portion of a lithium-ionbattery according to an exemplary embodiment.

FIG. 3 is a graph illustrating the battery voltage and the voltage ofthe positive and negative electrodes versus a lithium referenceelectrode during a battery formation process in which a charging voltageof 2.8 volts is used.

FIG. 4 is a graph illustrating two deep discharge cycles for a batterymanufactured using the formation process according to FIG. 3.

FIG. 5 is a graph illustrating the battery voltage and the voltage ofthe positive and negative electrodes versus a lithium referenceelectrode during a battery formation process in which a charging voltageof 3.4 volts is used.

FIG. 6 is a graph illustrating two deep discharge cycles for a batterymanufactured using the formation process according to FIG. 5.

FIG. 7 is a graph illustrating the resistance to capacity fade for thebatteries that were produced using the formation processes according toFIGS. 3 and 5.

FIG. 8 is a schematic view of a system in the form of an implantablemedical device implanted within a body or torso of a patient.

FIG. 9 is a schematic view of another system in the form of animplantable medical device.

DETAILED DESCRIPTION

An improved formation process for rechargeable batteries or cells (e.g.,lithium-ion batteries) has been developed that allows for the productionof batteries having improved resistance to capacity fade, particularlyunder deep-discharge conditions. The improved formation process utilizesa charging voltage that exceeds the charging voltage utilized inconventional battery formation processes, which in turn acts to producehigher levels of irreversible capacity loss during initial formation. Asa result, the zero volt crossing potential for the batteries will be ata higher voltage than would otherwise be possible, which may protect thepositive electrode active material from degradation that leads tocapacity fade in the battery.

Although the improved formation process may be applicable to batterieshaving a variety of configurations and chemistries, for simplicity theprocess will be described with respect to the batteries as described indetail below.

FIG. 2 is a schematic cross-sectional view of a portion of a battery 200according to an exemplary embodiment that includes at least one positiveelectrode 210 and at least one negative electrode 220. The size, shape,and configuration of the battery may be selected based on the desiredapplication or other considerations. For example the electrodes may beflat plate electrodes, wound electrodes, or folded electrodes (e.g.,Z-fold electrodes). According to other exemplary embodiments, thebattery may be a button cell battery, a thin film solid state battery,or another type of lithium-ion battery.

According to an exemplary embodiment, the battery 200 has a rating ofbetween approximately 1 and 1000 milliampere hours (mAh). According toanother exemplary embodiment, the battery has a rating of betweenapproximately 100 and 400 mAh. According to another exemplaryembodiment, the battery is an approximately 300 mAh battery. Accordingto another exemplary embodiment, the battery is an approximately 75 mAhbattery. According to another exemplary embodiment, the battery is anapproximately 10 mAh battery.

The battery case or housing (not shown) is formed of a metal or metalalloy such as aluminum or alloys thereof, titanium or alloys thereof,stainless steel, or other suitable materials. According to anotherexemplary embodiment, the battery case may be made of a plastic materialor a plastic-foil laminate material (e.g., an aluminum foil providedintermediate a polyolefin layer and a polyester layer).

An electrolyte is provided intermediate or between the positive andnegative electrodes to provide a medium through which lithium ions maytravel. The electrolyte may be a liquid (e.g., a lithium salt dissolvedin one or more non-aqueous solvents). According to an exemplaryembodiment, the electrolyte may be a mixture of ethylene carbonate (EC),ethylmethyl carbonate (EMC) and a 1.0 M salt of LiPF₆. According toanother exemplary embodiment, an electrolyte may be used that usesconstituents that may commonly be used in lithium batteries (e.g.,propylene carbonate, dimethyl carbonate, vinylene carbonate, lithiumbis-oxalatoborate salt (sometimes referred to as LiBOB), etc.).

Various other electrolytes may be used according to other exemplaryembodiments. According to an exemplary embodiment, the electrolyte maybe a lithium salt dissolved in a polymeric material such aspoly(ethylene oxide) or silicone. According to another exemplaryembodiment, the electrolyte may be an ionic liquid such asN-methyl-N-alkylpyrrolidinium bis(trifluoromethanesulfonyl)imide salts.According to another exemplary embodiment, the electrolyte may be a 3:7mixture of ethylene carbonate to ethylmethyl carbonate (EC:EMC) in a 1.0M salt of LiPF₆. According to another exemplary embodiment, theelectrolyte may include a polypropylene carbonate solvent and a lithiumbis-oxalatoborate salt. According to other exemplary embodiments, theelectrolyte may comprise one or more of a PVDF copolymer, aPVDF-polyimide material, and organosilicon polymer, a thermalpolymerization gel, a radiation cured acrylate, a particulate withpolymer gel, an inorganic gel polymer electrolyte, an inorganicgel-polymer electrolyte, a PVDF gel, polyethylene oxide (PEO), a glassceramic electrolyte, phosphate glasses, lithium conducting glasses,lithium conducting ceramics, and an inorganic ionic liquid gel, amongothers.

A separator 250 is provided intermediate or between the positiveelectrode 210 and the negative electrode 220. According to an exemplaryembodiment, the separator 250 is a polymeric material such as apolypropylene/polyethelene copolymer or another polyolefin multilayerlaminate that includes micropores formed therein to allow electrolyteand lithium ions to flow from one side of the separator to the other.The thickness of the separator 250 is between approximately 10micrometers (μm) and 50 μm according to an exemplary embodiment.According to a particular exemplary embodiment, the thickness of theseparator is approximately 25 μm and the average pore size of theseparator is between approximately 0.02 μm and 0.1 μm.

The positive electrode 210 includes a current collector 212 made of aconductive material such as a metal. According to an exemplaryembodiment, the current collector 212 comprises aluminum or an aluminumalloy.

According to an exemplary embodiment, the thickness of the currentcollector 212 is between approximately 5 μm and 75 μm. According to aparticular exemplary embodiment, the thickness of the current collector212 is approximately 20 μm. It should also be noted that while thepositive current collector 212 has been illustrated and described asbeing a thin foil material, the positive current collector may have anyof a variety of other configurations according to various exemplaryembodiments. For example, the positive current collector may be a gridsuch as a mesh grid, an expanded metal grid, a photochemically etchedgrid, or the like.

The current collector 212 has a layer of active material 216 providedthereon (e.g., coated on the current collector). While FIG. 3 shows thatthe active material 216 is provided on only one side of the currentcollector 212, it should be understood that a layer of active materialsimilar or identical to that shown as active material 216 may beprovided or coated on both sides of the current collector 212.

According to an exemplary embodiment, the active material 216 is amaterial or compound that includes lithium. The lithium included in theactive material 216 may be doped and undoped during discharging andcharging of the battery, respectively. According to an exemplaryembodiment, the active material 216 is lithium cobalt oxide (LiCoO₂).According to other exemplary embodiments, the active material may beprovided as one or more additional materials. For example, the activematerial may be LiMn₂O₄ or a material having the formulaLiCo_(x)Ni_((1−x))O₂, where x is between approximately 0.05 and 0.8.According to another exemplary embodiment, the active material is amaterial of the form LiNi_(x)Co_(y)Mn_((1−x−y))O₂ (e.g.,LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂). According to another exemplaryembodiment, the active material 216 is a metal-doped variety of one ofthe aforementioned materials, such as a material of the formLiM_(x)Co_(y)Ni_((1−x−y))O₂, where M is aluminum or titanium and x isbetween approximately 0.05 and 0.3 and y is between approximately 0.1and 0.3.

For certain applications, it may be desirable to provide a batteryhaving a cell voltage of greater than approximately 3 volts. In suchcases, a higher-voltage active material may be utilized on the positivecurrent collector, such as a material in the formLi_(2−x)Co_(y)Fe_(z)Mn_(4−(y+z))O₈ (e.g.,Li₂Co_(0.4)Fe_(0.4)Mn_(3.2)O₈). It is believed that such an activematerial may charge up to 5.2 volts versus a lithium referenceelectrode, making it possible to obtain an overall cell voltage of up toapproximately 3.7 volts. Other relatively high-voltage active materialsthat may be used for the positive electrode include LiCoPO₄; LiNiPO₄;Li₂CoPO₄F; Li[Ni_(0.2)Li_(0.2)Mn₆]O₂; and LiCo_(x)Mn_(2−x)O₄ (e.g.,LiCo_(0.3)Mn_(1.7)O₄).

According to various other exemplary embodiments, the active materialmay include a material such as a material of the form Li_(1−x)MO₂ whereM is a metal (e.g., LiCoO₂, LiNiO₂, and LiMnO₂), a material of the formLi_(1−W)(M′_(x)M″_(y))O₂ where M′ and M″ are different metals (e.g.,Li(Cr_(x)Mn_(1−x))O₂, Li(Al_(x)Mn_(1−w))O₂, Li(Co_(x)M_(1−x))O₂ where Mis a metal, Li(Co_(x)Ni_(1−x))O₂, and Li(Co_(x)Fe_(1−x))O₂)), a materialof the form Li_(1−w)(Mn_(x)Ni_(y)CO_(z))₂ (e.g.,Li(Mn_(1/3)Ni_(1/3)Co_(1/3))O₂, Li(Mn_(1/3)Ni_(1/3)Co_(1/3−x)Mg_(x))O₂,Li(Mn_(0.4)Ni_(0.4)Co_(0.2))O₂, and Li(Mn_(0.1)Ni_(0.1)Co_(0.8))O₂), amaterial of the form Li_(1−W)(Mn_(x)Ni_(x)Co_(1−2x))O₂, a material ofthe form Li_(1−w)(Mn_(x)Ni_(y)Co_(Z)Al_(w))O₂, a material of the formLi_(1−w)(Ni_(x)Co_(y)Al_(z))O₂ (e.g., Li(Ni_(0.8)Co_(0.15)Al_(0.05))O₂),a material of the form Li_(1−w)(Ni_(x)Co_(y)M_(z))O₂ where M is a metal,a material of the form Li_(1−W)(Ni_(x)Mn_(y)M_(z))O₂ where M is a metal,a material of the form Li(Ni_(x−y)Mn_(y)Cr_(2−x))O₄, LiMn₂O₄, a materialof the form LiM′M″₂O₄ where M′ and M″ are different metals (e.g.,LiMn_(2−y−z)Ni_(y)O₄, Li_(z)O₄, LiNiCuO₄, LiMn_(1−x)Al_(x)O₄,LiNi_(0.5)Ti_(0.5)O₄, and Li_(1.05)Al_(0.1)Mn_(1.85)O_(4−z)F_(z)),Li₂MnO₃, a material of the form Li_(x)V_(y)O_(z) (e.g., LiV₃O₈, LiV₂O₅,and LiV₆O₁₃), a material of the form LiMPO₄ where M is a metal orLiM_(x)′M″_(1−x)PO₄ where M′ and M″ are different metals (e.g., LiFePO₄,LiFe_(x)M_(1−x)PO₄ where M is a metal, LiVOPO₄, and Li₃V₂(PO₄)₃, andLiMPO_(4x) where M is a metal such as iron or vanadium and X is ahalogen such as fluorine, and combinations thereof.

A binder material may also be utilized in conjunction with the layer ofactive material 216 to bond or hold the various electrode componentstogether. For example, according to an exemplary embodiment, the layerof active material may include a conductive additive such as carbonblack and a binder such as polyvinylidine fluoride (PVDF) or anelastomeric polymer.

According to an exemplary embodiment, the thickness of the layer ofactive material 216 is between approximately 0.1 μm and 3 mm. Accordingto another exemplary embodiment, the thickness of the layer of activematerial 216 is between approximately 25 μm and 300 μm. According to aparticular exemplary embodiment, the thickness of the layer of activematerial 216 is approximately 75 μm.

The negative electrode 220 includes a current collector 222 that is madeof a conductive material such as a metal. According to an exemplaryembodiment, the current collector 222 is aluminum or an aluminum alloy.One advantageous feature of utilizing an aluminum or aluminum alloycurrent collector is that such a material is relatively inexpensive andmay be relatively easily formed into a current collector. Otheradvantageous features of using aluminum or an aluminum alloy includesthe fact that such materials may have a relatively low density, arerelatively highly conductive, are readily weldable, and are generallycommercially available. According to another exemplary embodiment, thecurrent collector 222 is titanium or a titanium alloy. According toanother exemplary embodiment, the current collector 222 is silver or asilver alloy.

While the negative current collector 222 has been illustrated anddescribed as being a thin foil material, the negative current collectormay have any of a variety of other configurations according to variousexemplary embodiments. For example, the positive current collector maybe a grid such as a mesh grid, an expanded metal grid, a photochemicallyetched grid, a metallized polymer film, or the like.

According to an exemplary embodiment, the thickness of the currentcollector 222 is between approximately 100 nm and 100 μm. According toanother exemplary embodiment, the thickness of the current collector 222is between approximately 5 μm and 25 μm. According to a particularexemplary embodiment, the thickness of the current collector 222 isapproximately 10 μm.

The negative current collector 222 has an active material 224 providedthereon. While FIG. 3 shows that the active material 224 is provided ononly one side of the current collector 222, it should be understood thata layer of active material similar or identical to that shown may beprovided or coated on both sides of the current collector 222.

According to an exemplary embodiment, the negative active material isselected such that it has an average potential that is greater or equalto approximately 0.2 V versus Li/Li⁺ (e.g., according to one particularexemplary embodiment, the negative active material has an averagepotential that is greater or equal to approximately 0.3 V versus Li/Li⁺;according to a particularly preferred embodiment, the negative activematerial is a titanate material having an average potential that isgreater or equal to approximately 1.5 V versus Li/Li⁺). The inventorshave unexpectedly discovered that the use of negative electrodematerials that possess a relatively high average potential versus Li/Li⁺reduces the likelihood of lithium plating. According to one exemplaryembodiment, such a negative active material is used in conjunction witha positive active material that has an average potential of greater thanapproximately 3 V versus Li/Li⁺ (e.g., LiCoO₂).

According to an exemplary embodiment, the negative active material 224is a lithium titanate material such as Li₄Ti₅O₁₂ (sometimes referred toas Li_(1+x)[Li_(1/3)Ti_(5/3)]O₄, with 0≦x<1). Other lithium titanatematerials which may be suitable for use as the negative active materialmay include one or more of the following lithium titanate spinelmaterials: H_(x)Li_(y−x)TiO_(x)O₄, H_(x)Li_(y−x)TiO_(x)O₄,Li₄M_(x)Ti_(5−x)O₁₂, Li_(x)Ti_(y)O₄, Li_(x)Ti_(y)O₄,Li₄[Ti_(1.67)Li_(0.03−y)M_(y)]O₄, Li₂TiO₃, Li₄Ti_(4.75)V_(0.25)O₁₂,Li₄Ti_(4.75)Fe_(0.25)O_(11.88), Li₄Ti_(4.5)Mn_(0.5)O₁₂, and LiM′M″XO₄(where M′ is a metal such as nickel, cobalt, iron, manganese, vanadium,copper, chromium, molybdenum, niobium, or combinations thereof, M″ is anoptional three valent non-transition metal, and X is zirconium,titanium, or a combination of these two). Note that such lithiumtitanate spinel materials may be used in any state of lithiation (e.g.,Li_(4+x)Ti₅O₁₂, where 0≦x≦3).

According to an exemplary embodiment, the lithium titanate may beprovided such that at least five percent is in the form of lithiumtitanate nanoparticles (e.g., having a particle size of less thanapproximately 500 nanometers). The use of such nanoparticles is intendedto provide greater surface area for doping and undoping of lithium ions.

According to other exemplary embodiments, a lithium vanadate (e.g.,Li_(1.1)V_(0.9)O₂) material may be used as the negative active material.Other materials having cycling potentials that exceed that of lithium byseveral hundred millivolts and which may be suitable for use as thenegative active material include the materials listed in Table 1. Suchmaterials may be used alone or in combination with the lithium titanatesdescribed above and/or any of the other materials listed in Table 1.

TABLE 1 Cycling Potentials (vs Li) Class Compound Vmin Vmax Vavg OxidesTiO₂ (Anatase) 1.4 2 1.80 Oxides WO₂ 0.6 1.3 0.80 Oxides WO₃ 0.5 2.6 1.0Oxides MoO₂ 1.3 2 1.60 Oxides Nb₂O₅ 1.0 2 1.50 Oxides LiWO₂ 0.75 OxidesLi_(x)MoO₂ 0.8 2 1.60 Oxides V₆O₁₃ 2.30 Oxides Li₆Fe₂O₃ 0.75 OxidesLiFeO₂ 1.0 3.0 2.0 Oxides Fe₂O₃ 0.2 2.0 0.75 Oxides MO where M = Co, Ni,Cu or Fe 0.8-1.5 Sulfides FeS₂ 1.3 1.9 1.65 Sulfides MoS₂ 1.75 SulfidesTiS₂ 2.00 Alloys Sn—Bi 0.75 Alloys Alloys comprising of Al, Si or 0.30Sn and other elements Alloys Sn—Co—C 0.30 Alloys Sb 0.90 Alloys NbSe₃1.95 Alloys Bi 0.80 Alloys In 0.60 Alloys LixAl 0.36 Alloys LixSn 0 1.30.50 Alloys Sn—Sb 0.0-1.0 Polymers Poly(phenylquinoline) 1.50 PolymersPolyparaphenylene 0.70 Polymers Polyacetylene 1.00 Vanadates Li_(x)MVO₄where M = Ni, Co, 2.0-0.5 Cd, Zn

A binder material may also be utilized in conjunction with the layer ofactive material 224. For example, according to an exemplary embodiment,the layer of active material may include a binder such as polyvinylidinefluoride (PVDF) or an elastomeric polymer. The active material 224 mayalso include a conductive material such as carbon (e.g., carbon black)at weight loadings of between zero and ten percent to provide increasedelectronic conductivity.

According to various exemplary embodiments, the thickness of the activematerial 224 is between approximately 0.1 μm and 3 mm. According toother exemplary embodiments, the thickness of the active material 224may be between approximately 25 μm and 300 μm. According to anotherexemplary embodiment, the thickness of the active material 224 may bebetween approximately 20 μm and 90 μm, and according to a particularexemplary embodiment, approximately 75 μm.

Lithium plating occurs when the potential of the negative electrodeversus a lithium reference electrode reaches 0 volts, and is awell-known phenomenon that can lead to loss in performance oflithium-ion batteries. When used in a negative electrode of alithium-ion battery, lithium titanate active materials cycle lithium ata potential plateau of about 1.55 volts (which is substantially higherthan graphitic carbon, which cycles lithium at approximately 0.1 voltsin the fully charged state). As a result, batteries using lithiumtitanate as a negative active material are less susceptible to lithiumplating than those using carbon-based materials for the negative activematerial.

One potential advantageous feature of utilizing a negative electrodeactive material such as a lithium titanate material or another materialhaving an average potential that is greater or equal to approximately0.2 V versus Li/Li⁺ is that more favorable design rules may be possible.One such design rule relates to the cell balance parameters.

Cell balance refers to the ratio of the negative electrode capacity tothe positive electrode capacity. Thus, the cell balance γ (gamma) canthus be expressed as an equation having the form:

$\gamma = \frac{Q_{neg}}{Q_{pos}}$where Q_(neg) is equal to the product of the amount of negativeelectrode material, the percentage of the active material contributingto the capacity, and the specific capacity of the active material andQ_(pos) is equal to the product of the amount of positive material, thepercent of the active material contributing to the capacity, and thespecific capacity of active material. According to an exemplaryembodiment in which a lithium titanate negative active material and alithium cobalt oxide positive active material are used, the specificcapacity of the negative active material may be 165 mAh/g (at C/3-C/1,between 1.2 and 2.0 volts versus Li) and the specific capacity of thepositive active material may be 150 mAh/g (at C/3-C/1, between 4.25 and3.0 volts versus Li).

Cell balance dictates the mass (deposition) ratio of the two electrodes.In conjunction with charge cutoff voltage, cell balance determines whatfraction of the active Li sites in the two electrode materials isutilized during charge and discharge. For example, in a negative-limiteddesign, nearly all active Li sites in the negative material are utilizedin the negative electrode, but only a fraction of the active sites inthe positive electrode are utilized, and vice versa. The chosen cellbalance hence determines the specific capacity delivered by bothelectrodes, and the stability of their performance for repeated cycling.Primary considerations for choosing the cell balance include safety,delivered capacity and capacity fade under regular cycling and deepdischarge conditions.

Lithium-ion batteries using carbon-based negative active materials aretypically fabricated such that the capacity of the electrodes isbalanced (i.e., the capacities of the positive electrode and negativeelectrode are equal) or positive-limited (i.e., the capacity of thepositive electrode is less than that of the negative electrode). Thereason for this design rule is that if the battery was negative-limited,the voltage of the negative electrode versus a lithium referenceelectrode would drop to near zero volts during charging of the battery,which may result in lithium plating (since carbonaceous negativeelectrodes typically operate very close to potential of metallic Li(0.1-0.2 volts versus Li), and a further decrease in potential at thenegative electrode would result in plating of lithium).

According to an exemplary embodiment in which a battery utilizes anegative electrode active material such as a lithium titanate materialor another material having an average potential that is greater or equalto approximately 0.2 volts versus a lithium reference electrode, thebattery may be fabricated with a negative-limited design in which thecapacity of the negative electrode is less than that of the positiveelectrode. For example, for a lithium titanate active material having apotential plateau of approximately 1.55 volts, a cell balance of betweenapproximately 0.80 to 0.90 may be used (i.e., the nominal capacity ofthe negative electrode may be between 80 and 90 percent of the nominalcapacity of the positive electrode). According to another exemplaryembodiment, the cell balance may be less than approximately 0.8 (e.g.,0.78).

Batteries such as those described herein having a negative-limiteddesign may provide a number of advantages. For example, when chargedusing the formation process described below, such batteries may exhibitlower capacity fade and lower power fade due to reduced likelihood ofover-charging of the positive electrode. Other advantages include theability to more rapidly recharge the battery without riskingpolarization of the positive electrode to high potentials, which alsohelps reduce capacity fade. Further, with a negative limited design,additives which prevent degradation due to oxidation of the positiveelectrode are not required. This not only reduces cost, but also makesthe electrolyte chemistry simpler. Such additives typically lead to gasformation and swelling when oxidized, and all of that is eliminated inthe negative limited design. Another advantage of a negative limiteddesign over a positive limited design, which may depend in part on thespecific choice of materials used, is that negative limited designoffers higher energy density. For example, a lithium titanate materialused for a negative has a lower density than a lithium cobalt oxidepositive active material—with a negative limited cell design, theproportion of the negative material required is less, which offersbetter volumetric efficiency.

One particular advantage associated with negative-limited batteries(using negative active materials such as a lithium titanate material oranother material having an average potential that is greater or equal toapproximately 0.2 volts versus a lithium reference electrode) isdescribed with respect to FIGS. 3-7. According to an exemplaryembodiment, an improved formation process may be used in conjunctionwith such batteries that provides enhanced tolerance to repeated deepdischarge conditions, as evidenced, for example, by improved resistanceto capacity fade.

During a formation (i.e., initial charging of the battery) process,lithium-ion batteries are charged at a relatively low rate (such as C/10or slower) to the maximum operating voltage of the battery. By way ofexample, for a lithium-ion battery (LiCoO₂ positive active material,lithium titanate negative active material) that is configured to have anintended operating voltage range between approximately 2.8 volts (fullcharge) and 1.8 volts (discharge cut-off voltage), a formation processmight involve charging the battery at a voltage of 2.8 volts at a rateof C/10 and then holding the battery at the 2.8 volt level for fourhours. FIG. 3 illustrates the voltage behavior for a battery undergoingsuch a formation process, and includes a curve 310 representative of theoverall battery voltage, a curve 312 representative of the positiveelectrode potential versus a lithium reference electrode, and a curve314 representative of the negative electrode potential versus a lithiumreference electrode. As shown in FIG. 3, the overall battery voltagegradually increases to a 2.8 volt plateau 311 at the end of formationprocess, while the negative electrode potential versus a lithiumreference electrode drops to a level of between approximately 1.3 and1.4 volts toward the end of curve 314.

The formation process described in the preceding paragraph may result ina relatively low irreversible capacity loss for the battery (e.g.,between approximately 6 and 11 percent) during the formation process.This irreversible capacity loss results primarily from the passivationof the two electrodes, and occurs when otherwise cyclable lithium ionsin the battery (i.e., lithium ions that shuttle between the positive andnegative electrodes during charging and discharging) combine with anelectrode active material.

FIG. 4 illustrates the voltage behavior for the battery when it issubjected to two deep discharge cycles where the battery is dischargedto zero volts, and includes a curve 320 representative of the overallbattery voltage, a curve 322 representative of the positive electrodepotential versus a lithium reference electrode, and a curve 324representative of the negative electrode potential versus a lithiumreference electrode. As the overall battery voltage drops toward zerovolts (i.e., a deep discharge condition), the curves 322, 324representing the potentials of the positive and negative electrodesconverge toward and intersect at a point 326 referred to as the zerovolt crossing potential for the battery. As illustrated in FIG. 4, thezero voltage crossing potential for this battery occurs at a point wherethe positive electrode potential versus a lithium reference electrode isless than 2.0 volts (e.g., between approximately 1.6 and 1.8 volts).

FIG. 7 is a graph that includes a curve 330 representative of thedischarge capacity of the battery as it undergoes repeated deepdischarge cycles. The variations in the curves at cycles 50 and 100 area result of two characterization cycles that were run during the test.These characterization cycles consist of (1) a slow rate cycle todetermine trends in full charge/discharge capacity, (2) an applicationrate cycle to determine impact of repeated deep discharge on performancein normal application conditions. Other smaller variations (such asaround cycle 170) are experimental artifacts.

As illustrated by the curve 330 shown in FIG. 7, the discharge capacityof the battery drops as it is subjected to repeated deep dischargecycles. This capacity fade with repeated deep discharge cycles resultsin a battery that must be recharged more frequently, since the batterydoes not hold as much charge as it did after initial formation.

The inventors have determined through experimentation that one factorthat contributes to the capacity fade is that the positive activematerial (in this case LiCoO₂) tends to relatively rapidly degrade whendriven to a potential versus a lithium reference electrode that is lessthan 2.0 volts (e.g., approximately 1.6 volts). It should be noted thatwhere different positive active materials are used, degradation of suchmaterials may occur at a different level. As illustrated in FIG. 4, thepotential of the positive electrode versus a lithium referenceelectrode, as represented by curve 322, drops to a level that is below2.0 volts as the battery is discharged to a near-zero-volt charge stateand the curves 322, 324 approach the zero volt crossing potential 326for the battery.

The inventors have surprisingly discovered that the resistance tocapacity fade for a negative-limited battery that utilizes a lithiumtitanate active material on the negative electrode can be significantlyimproved by using a formation process in which the charging voltage isgreater than the normal fully charged voltage of the battery, with noassociated increase in charging rate (e.g., the same C/10 charging ratemay be used as described in the preceding example).

According to an exemplary embodiment, a lithium-ion battery (LiCoO₂positive active material, lithium titanate negative active material)having an intended operating voltage range from 2.8 volts (fully chargedstate) to 1.8 volts (cut-off voltage) is subjected to a formationprocess that utilizes a charging voltage of between approximately 3.4volts and 3.8 volts at a C/10 charge rate, after which the batteryvoltage is held at this level for a period of between approximately 4and 12 hours. FIG. 5 illustrates the voltage behavior for a batteryundergoing such a formation process, and includes a curve 340representative of the overall battery voltage, a curve 342representative of the positive electrode potential versus a lithiumreference electrode, and a curve 344 representative of the negativeelectrode potential versus a lithium reference electrode.

As shown in FIG. 5, the battery voltage gradually increases to a 3.4volt plateau 341 at the end of formation process. The higher chargingvoltage during the formation process results in a greater amount ofirreversible capacity loss as compared to formation process using alower charging voltage (e.g., 2.8 volts). For example, the totalirreversible capacity loss obtained using the higher-voltage formationprocess is expected to be between approximately 12 and 20 percent,depending on factors including, for example, the selected cut-offvoltage and hold duration.

The negative electrode potential versus a lithium reference electrodedrops to a level below approximately 0.9 volts (e.g., betweenapproximately 0.5 and 0.8 volts) as the battery reaches its fullycharged state (i.e., the charge top-off voltage during normal use),while the positive electrode potential does not exhibit a large voltageincrease. This is primarily due to the fact that the battery isnegative-limited such that the capacity of the negative electrode isless than that of the positive electrode (i.e., the negative electrodedepletes before the positive electrode, which results in a potentialdrop for the negative electrode).

As the potential of the negative electrode drops below approximately 0.9volts, a reaction takes place in which the solvent component of theelectrolyte (e.g., ethylene carbonate) reduces. In this reductionreaction, a passive film (e.g., lithium carbonate or lithium alkylcarbonate) is formed on the negative electrode. Additionally, anadditive such as carbon or a carbon-based material may be provided inthe negative active material to assist in the formation of the passivefilm and to increase the amount of lithium that reacts irreversibly toform the film (e.g., between approximately 5 and 10 volume percentcarbon may be provided in the lithium titanate material).

One advantageous feature of the reduction reaction is that the increasedirreversible capacity loss of the battery tends to increase the zerovolt crossing potential for the battery, which allows for enhancedresistance to capacity fade since the higher zero volt crossingpotential is above the level where the positive active materialdegrades.

FIG. 6 illustrates the voltage behavior for such a battery when it issubjected to two deep discharge cycles where the battery is dischargedto zero volts, and includes a curve 350 representative of the overallbattery voltage, a curve 352 representative of the positive electrodepotential versus a lithium reference electrode, and a curve 354representative of the negative electrode potential versus a lithiumreference electrode. As the overall battery voltage drops toward zerovolts (i.e., a deep discharge condition), the curves 352, 354representing the potentials of the positive and negative electrodesconverge toward and intersect at the zero volt crossing potential 356for the battery. As illustrated in FIG. 6, the zero voltage crossingpotential for this battery occurs at a point where the positiveelectrode potential versus a lithium reference electrode is greater thanapproximately 3.5 volts, which is above the threshold at which thepositive active material (LiCoO₂) would be expected to degrade. Itshould also be noted that the lithium titanate negative electrode activematerial is also stable at this potential.

The battery produced using the high-voltage formation process exhibitssignificantly enhanced resistance to capacity fade when subjected torepeated deep discharge cycles, as illustrated by the curve 360 in FIG.7. The capacity of the battery is substantially constant overapproximately 170 deep discharge cycles, in contrast to the relativelysignificant capacity fade exhibited by the battery using a moreconventional formation process (curve 330).

Accordingly, the use of a voltage that exceeds the voltage of the fullycharged battery during the formation process acts to induce a largeramount of irreversible capacity loss in the battery than would otherwisebe obtained. This in turn increases the zero voltage crossing potentialfor the battery above the level where the positive active material woulddegrade. As a result, the resistance to capacity fade with repeated deepdischarge cycles is enhanced.

Example 1

Lithium ion cells having nominal capacities of approximately 0.15 amperehours (Ah) re prepared having a lithium cobalt oxide (LiCoO₂) activematerial on the positive electrode and a lithium titanate activematerial on the negative electrode. The electrolyte used included amixture of ethylene carbonate, ethylmethyl carbonate, and 1 molar LiPF₆as the lithium salt. The cells were constructed as spirally woundprismatic cells hermetically sealed in a stainless steel can with aglass feedthrough.

The negative and positive electrodes for the cells were prepared via aslurry coating and calendaring process. Both electrodes included theaforementioned active material, a conductive carbon additive, andorganic binder. The mass loading (grams of materials per unit area)during the slurry coating process was controlled on both electrodes toattain a mass ratio of the two active materials to be 0.71. Based on thenominal specific capacities of the two active materials, determined viahalf cell testing (150 mAh/g for lithium cobalt oxide, 165 mAh/g forlithium titanate), the cell balance of this cell was 0.78 (i.e., thenominal capacity of the negative electrode was 78% of the nominalcapacity of the positive electrode).

Cells built with the above design were subjected to a conventionalformation process in which the cells were charged at C/10 (15 mA) to thenormal cut-off voltage of 2.8 volts and were held potentiostatically at2.8 volts for 4 hours. Another group of cells built with the abovedesign were subjected to an improved high voltage formation process. Inthis process, the cells were charged at C/10 (15 mA) to 3.4 volts andwere held potentiostatically at 3.4 volts for 4 hours. After theserespective formation processes, both groups of cells were discharged atC/10 to 1.8 volts and held at 1.8 volts for 4 hours.

The capacities delivered by the cells were measured both during theformation charge process and during the discharge process. The measuredcapacity values are given in Table 2. The formation charge capacity istypically greater than the discharge capacity, which is indicative ofthe irreversible capacity loss accounted for by the irreversibleprocesses. The cells show 6.5% irreversible capacity during theconventional formation process, compared to 12.2% in the improved highvoltage formation process. Thus, the difference in the irreversiblecapacities is approximately 5.7% between the 2.8 volts formation andhigh voltage 3.4 volts formation. This difference is seen to increase asa greater voltage and/or a longer potentiostatic hold duration ischosen. It is noteworthy that this greater irreversible capacity doesnot reduce the reversible capacity of the cell. Cycle 1 dischargecapacity is the same for both groups of cells, as shown in Table 2. Thehigher irreversible capacity is attained by extending the voltage windowbeyond its normal cut-off and thus comes out from the extended capacityand not from the nominal capacity over the voltage range for normalcycling. Thus, unlike typical cases where higher irreversible capacitymeans lower irreversible capacity, such a lowering of the reversiblecapacity was not observed.

TABLE 2 Cycle 1 Cycle 1 Cycle 160 Cycle 160 % Fade charge dischargecharge discharge between capacity Capacity capacity capacity cycles 1and (Ah) (Ah) (Ah) (Ah) 160 Cells 0.168 0.157 0.081 0.081 48.4% formedat 2.8 volts Cells 0.180 0.158 0.152 0.152 3.5% formed at 3.4 volts

These cells were then cycled between 2.8 volts to 0.0 volts to determinethe stability of performance for repeated deep discharge to zero volts.During the charge phase of this cycling, the cells were taken to 2.8volts at a 1 C rate (150 mA) and potentiostatically held at 2.8 voltsfor 1 hour. During the discharge phase of the cycling, the cells weretaken to 0.0 volts in a staged manner—first the cells were discharged to1.8 volts at 150 mA, held potentiostatically at 1.8 volts for 1 hour,then discharged to 0.0 volts at 0.1 mA and finally heldpotentiostatically at 0.0 volts for 24 hours. The difference inperformance of the two groups of cells in this test is shown in FIG. 7,where the capacity obtained in every cycle as a percent of the initialcapacity is plotted versus the cycle number. The cells with theconventional formation show large capacity fade, with the capacityfalling to approximately 52% after 160 deep discharge cycles. In sharpcontrast, the cells with the high voltage formation continue to showgreater than 95% of the initial capacity after 160 deep dischargecycles.

The batteries and formation methods described in the preceding Example 1and elsewhere in the present application may find utility in a varietyof applications, including in implantable medical devices (IMDs). FIG. 8illustrates a schematic view of a system 400 (e.g., an implantablemedical device) implanted within a body or torso 432 of a patient 430.The system 400 includes a device 410 in the form of an implantablemedical device that for purposes of illustration is shown as adefibrillator configured to provide a therapeutic high voltage (e.g.,700 volt) treatment for the patient 430.

The device 410 includes a container or housing 414 that is hermeticallysealed and biologically inert according to an exemplary embodiment. Thecontainer may be made of a conductive material. One or more leads 416electrically collect the device 410 and to the patient's heart 420 via avein 422. Electrodes 417 are provided to sense cardiac activity and/orprovide an electrical potential to the heart 420. At least a portion ofthe leads 416 (e.g., an end portion of the leads shown as exposedelectrodes 417) may be provided adjacent or in contact with one or moreof a ventricle and an atrium of the heart 420.

The device 410 includes a battery 440 provided therein to provide powerfor the device 410. The size and capacity of the battery 440 may bechosen based on a number of factors, including the amount of chargerequired for a given patient's physical or medical characteristics, thesize or configuration of the device, and any of a variety of otherfactors. According to an exemplary embodiment, the battery is a 5 mAhbattery. According to another exemplary embodiment, the battery is a 300mAh battery. According to various other exemplary embodiments, thebattery may have a capacity of between approximately 1 and 1000 mAh.

According to other exemplary embodiments, more than one battery may beprovided to power the device 410. In such exemplary embodiments, thebatteries may have the same capacity or one or more of the batteries mayhave a higher or lower capacity than the other battery or batteries. Forexample, according to an exemplary embodiment, one of the batteries mayhave a capacity of approximately 500 mAh while another of the batteriesmay have a capacity of approximately 75 mAh.

According to an exemplary embodiment, the battery may be configured suchthat it may be charged and recharged using an inductive charging systemin which a primary or external coil is provided at an exterior surfaceof a portion of the body (either proximate or some distance away fromthe battery) and a secondary or internal coil is provided below the skinadjacent the primary coil.

According to another exemplary embodiment shown in FIG. 9, animplantable neurological stimulation device 500 (an implantable neurostimulator or INS) may include a battery 502 such as those describedabove with respect to the various exemplary embodiments. Examples ofsome neuro stimulation products and related components are shown anddescribed in a brochure titled “Implantable Neturostinmulation Systems”available from Medtronic, Inc.

An INS generates one or more electrical stimulation signals that areused to influence the human nervous system or organs. Electricalcontacts carried on the distal end of a lead are placed at the desiredstimulation site such as the spine or brain and the proximal end of thelead is connected to the INS. The INS is then surgically implanted intoan individual such as into a subcutaneous pocket in the abdomen,pectoral region, or upper buttocks area. A clinician programs the INSwith a therapy using a programmer. The therapy configures parameters ofthe stimulation signal for the specific patient's therapy. An INS can beused to treat conditions such as pain, incontinence, movement disorderssuch as epilepsy and Parkinson's disease, and sleep apnea. Additionaltherapies appear promising to treat a variety of physiological,psychological, and emotional conditions. Before an INS is implanted todeliver a therapy, an external screener that replicates some or all ofthe INS functions is typically connected to the patient to evaluate theefficacy of the proposed therapy.

The INS 500 includes a lead extension 522 and a stimulation lead 524.The stimulation lead 524 is one or more insulated electrical conductorswith a connector 532 on the proximal end and electrical contacts (notshown) on the distal end. Some stimulation leads are designed to beinserted into a patient percutaneously, such as the Model 3487APisces-Quad® lead available from Medtronic, Inc. of Mimneapolis Minn.,and stimulation some leads are designed to be surgically implanted, suchas the Model 3998 Specify® lead also available from Medtronic.

Although the lead connector 532 can be connected directly to the INS 500(e.g., at a point 536), typically the lead connector 532 is connected toa lead extension 522. The lead extension 522, such as a Model 7495available from Medtronic, is then connected to the INS 500.

Implantation of an INS 500 typically begins with implantation of atleast one stimulation lead 524, usually while the patient is under alocal anesthetic. The stimulation lead 524 can either be percutaneouslyor surgically implanted. Once the stimulation lead 524 has beenimplanted and positioned, the stimulation lead's 524 distal end istypically anchored into position to minimize movement of the stimulationlead 524 after implantation. The stimulation lead's 524 proximal end canbe configured to connect to a lead extension 522.

The INS 500 is programmed with a therapy and the therapy is oftenmodified to optimize the therapy for the patient (i.e., the INS may beprogrammed with a plurality of programs or therapies such that anappropriate therapy may be administered in a given situation).

A physician programmer and a patient programmer (not shown) may also beprovided to allow a physician or a patient to control the administrationof various therapies. A physician programmer, also known as a consoleprogrammer, uses telemetry to communicate with the implanted INS 500, soa clinician can program and manage a patient's therapy stored in the INS500, troubleshoot the patient's INS system, and/or collect data. Anexample of a physician programmer is a Model 7432 Console Programmeravailable from Medtronic. A patient programmer also uses telemetry tocommunicate with the INS 500, so the patient can manage some aspects ofher therapy as defined by the clinician. An example of a patientprogrammer is a Model 7434 Itrel® 3 EZ Patient Programmer available fromMedtronic.

According to an exemplary embodiment, a battery provided as part of theINS 500 may be configured such that it may be charged and rechargedusing an inductive charging system in which a primary or external coilis provided at an exterior surface of a portion of the body (eitherproximate or some distance away from the battery) and a secondary orinternal coil is provided below the skin adjacent the primary coil.

While the medical devices described herein (e.g., systems 400 and 500)are shown and described as a defibrillator and a neurologicalstimulation device, it should be appreciated that other types ofimplantable medical devices may be utilized according to other exemplaryembodiments, such as pacemakers, cardioverters, cardiac contractilitymodules, drug administering devices, diagnostic recorders, cochlearimplants, and the like for alleviating the adverse effects of varioushealth ailments.

It is also contemplated that the medical devices described herein may becharged or recharged when the medical device is implanted within apatient. That is, according to an exemplary embodiment, there is no needto disconnect or remove the medical device from the patient in order tocharge or recharge the medical device.

It is important to note that the construction and arrangement of thebatteries and cells and the methods for forming such batteries as shownand described in the various exemplary embodiments is illustrative only.Although only a few embodiments have been described in detail in thisdisclosure, those skilled in the art who review this disclosure willreadily appreciate that many modifications are possible withoutmaterially departing from the novel teachings and advantages of thesubject matter recited in the claims. Accordingly, all suchmodifications are intended to be included within the scope of thepresent invention as defined in the appended claims. The order orsequence of any process or method steps may be varied or re-sequencedaccording to other exemplary embodiments. Other substitutions,modifications, changes, and omissions may be made in the design,operating conditions, and arrangement of the various exemplaryembodiments without departing from the scope of the present inventionsas expressed in the appended claims.

What is claimed is:
 1. A method for producing a battery comprising:providing a battery comprising a positive electrode, a negativeelectrode, and an electrolyte comprising a solvent and a salt, whereinthe capacity of the negative electrode is less than that of the positiveelectrode and the negative electrode comprises an active material havingan average potential versus a lithium reference electrode of greaterthan approximately 1.0 volts; and applying an initial charge to thebattery and continuing the initial charge beyond the point where thebattery is fully charged, wherein the initial charge after the batteryis fully charged is applied for approximately 4 hours or more at avoltage that is greater than a fully charged voltage of the battery. 2.The method of claim 1, wherein step of applying an initial charge to thebattery is performed for a sufficient amount of time to cause thepotential of the negative electrode versus a lithium reference electrodeto drop to a level such that an irreversible reaction occurs between theelectrolyte and the negative electrode.
 3. The method of claim 2,wherein the irreversible reaction forms a solid electrolyte interphase(SEI) layer on the negative electrode.
 4. The method of claim 1, whereinthe fully charged voltage of the battery is between approximately 2.5and 3.0 volts and the step of applying an initial charge to the batteryutilizes a voltage of between approximately 3.4 and 3.8 volts.
 5. Themethod of claim 4, wherein the positive electrode comprises an activematerial and the step of applying an initial charge to the batteryincreases the zero volt crossing potential of the battery to a levelthat is greater than a decomposition potential of the active material ofthe positive electrode.
 6. The method of claim 5, wherein the zero voltcrossing potential of the battery after the step of applying the initialcharge to the battery is greater than approximately 3.0 volts.
 7. Themethod of claim 4, wherein the step of applying an initial charge to thebattery is performed for a sufficient amount of time to cause thepotential of the negative electrode versus a lithium reference electrodeto drop to a level below approximately 0.9 volts.
 8. The method of claim1, wherein step of applying an initial charge to the battery occurs at acharging rate of approximately C/10 or slower.
 9. The method of claim 1,wherein the negative electrode comprises an additive configured toenhance the formation of an SEI layer.
 10. The method of claim 9,wherein the additive comprises carbon.
 11. The method of claim 1,wherein the electrolyte comprises ethylene carbonate.
 12. The method ofclaim 1, wherein the active material of the negative electrode comprisesa lithium titanate material.
 13. The method of claim 12, wherein thepositive electrode comprises an active material comprising lithiumcobalt oxide.
 14. The method of claim 1, wherein between approximately12% and approximately 20% irreversible capacity loss is created duringthe step of applying an initial charge and continuing the initialcharge.
 15. The method of claim 1, wherein the initial charge is appliedafter the negative electrode is saturated with cyclable lithium at avoltage that is greater than the fully charged voltage of the battery.16. The method of claim 1, wherein after the battery is fully charged,application of the initial charge at the voltage that is greater thanthe fully charged voltage of the battery causes at least a portion ofthe solvent to undergo a reduction reaction.
 17. The method of claim 16,wherein a passive film is formed on the negative electrode during thereduction reaction.
 18. The method of claim 1, wherein during the stepof applying an initial charge to the battery and continuing the initialcharge, greater than approximately 12% irreversible capacity loss of thebattery is created.
 19. A method for producing a battery comprising:providing a battery comprising a positive electrode, a negativeelectrode, and an electrolyte comprising a solvent and a salt, whereinthe capacity of the negative electrode is less than that of the positiveelectrode and the negative electrode comprises an active materialcomprising lithium titanate; and applying an initial charge to thebattery and continuing the initial charge beyond the point where thebattery is fully charged, wherein the initial charge after the batteryis fully charged is applied for approximately 4 hours or more at avoltage that is greater than a fully charged voltage of the battery suchthat at least a portion of the solvent undergoes a reduction reactionand greater than approximately 12% irreversible capacity loss of thebattery is created during the step of applying an initial charge andcontinuing the initial charge.
 20. The method of claim 19, whereinduring a deep discharge, the potential of the negative electrode rapidlyincreases to a zero volt crossing potential that is greater than adecomposition potential of an active material of the positive electrode.21. The method of claim 20, wherein the zero volt crossing potential isgreater than approximately 3.0 volts.
 22. The method of claim 19, duringthe step of applying an initial charge to the battery and continuing theinitial charge, a portion of the solvent undergoes a reduction reactionto form a solid electrolyte interphase (SEI) layer on the negativeelectrode.
 23. The method of claim 19, wherein the positive electrodecomprises an active material comprising lithium cobalt oxide.
 24. Amethod for producing a battery comprising: providing a batterycomprising a positive electrode, a negative electrode, and anelectrolyte comprising a solvent and a salt, wherein the capacity of thenegative electrode is less than that of the positive electrode and thenegative electrode comprises an active material having an averagepotential versus a lithium reference electrode of greater thanapproximately 1.0 volts; and while the battery is fully charged,applying a charge to the battery at a voltage that is greater than afully charged voltage of the battery for a period of time greater thanor equal to approximately 4 hours.
 25. The method of claim 24, whereinthe period of time during which the charge is applied while the batteryis fully charged is less than or equal to approximately 12 hours. 26.The method of claim 24, wherein the charge applied while the battery isfully charged is greater than or equal to approximately 0.6 volts morethan a fully-charged voltage of the battery.
 27. The method of claim 26,wherein the charge applied while the battery is fully charged is betweenapproximately 3.4 volts and 3.8 volts.
 28. The method of claim 27,wherein the active material of the negative electrode is lithiumtitanate.
 29. The method of claim 28, wherein the positive electrodecomprises an active material comprising lithium cobalt oxide.